Recently, a nonaqueous electrolyte battery such as a lithium ion secondary battery has been actively researched and developed as a high energy-density battery. The nonaqueous electrolyte battery is anticipated as a power source for vehicles such as hybrid automobiles, electric cars, an uninterruptible power supply for base stations for portable telephone, or the like. Therefore, the nonaqueous electrolyte battery is demanded to, in addition to having a high energy density, be excellent in other performances such as rapid charge-and-discharge performances and long-term reliability, as well. For example, not only is the charging time remarkably shortened in a nonaqueous electrolyte battery capable of rapid charge and discharge, but the battery is also capable of improving motive performances, such as in hybrid automobiles, and efficient recovery of regenerative energy of power.
In order to enable rapid charge-and-discharge, electrons and Li ions must be able to migrate rapidly between the positive electrode and the negative electrode. However, when a battery using a carbon-based negative electrode is repeatedly subjected to rapid charge-and-discharge, precipitation of dendrite of metallic lithium on the electrode may sometimes occur, raising concern of heat generation or fires due to internal short circuits.
In light of this, a battery using a metal composite oxide in a negative electrode in place of a carbonaceous material has been developed. In particular, in a battery using an oxide of titanium in the negative electrode, rapid charge-and-discharge can be stably performed. Such a battery also has a longer life than in the case of using a carbon-based negative electrode.
However, compared to carbonaceous materials, oxides of titanium have a higher potential relative to metallic lithium. That is, oxides of titanium are more noble. Furthermore, oxides of titanium have a lower capacity per weight. Therefore, a battery using an oxide of titanium as the negative electrode active material has a problem that the energy density is lower. In particular, when a material having a high potential relative to metallic lithium is used as a negative electrode material, the voltage becomes lower than that of a conventional battery using a carbonaceous material. Therefore, when such a material is used for systems requiring a high voltage such as an electric vehicle and a large-scale electric power storage system, there is a problem that the number of batteries connected in a series becomes large.
The potential of the electrode using an oxide of titanium is about 1.5 V (vs. Li/Li+) relative to metallic lithium and is higher (more noble) than that of the negative electrode with carbonaceous material. The potential of an oxide of titanium arises from the redox reaction between Ti3+ and Ti4+ upon electrochemical insertion and extraction of Li, and is therefore electrochemically limited. It has therefore been conventionally difficult to drop the potential of the electrode in order to improve the energy density.